1. Field of the Invention
The present invention relates to an interrogator which employs a scheme which perform radio communication with a responder such as a Radio Frequency Identification (RFID) tag while changing communication frequencies with time, i.e., a so-called frequency hopping scheme.
2. Description of the Related Art
In recent years, a radio communication system called a RFID system attracts attention. This system is constituted by an interrogator and a compact responder. The responder includes an IC chip and an antenna, and is generally called a RFID tag, a radio tag, an electronic tag, or the like. The interrogator performs radio communication with the responder by using an electric wave or an electromagnetic wave to write and read data in a non-contact manner. The interrogator is generally called a reader/writer, or the like.
Some interrogators employ a frequency hopping scheme, which is a kind of spread spectrum scheme. The interrogator of this type changes frequencies at which communication is performed with time according to a preset frequency hopping pattern. This operation is called a frequency hopping operation.
In general, when a plurality of interrogators are arranged in a limited region, adjacent interrogators interfere with each other when the interrogators use the same frequency. However, when the frequency hopping scheme is employed, interference between the interrogators can be reduced. However, when the interrogators use the same frequency hopping pattern while shifting phases, adjacent interrogators may perform hopping operations at the same frequency at the same time. In this case, the two adjacent interrogators may continuously use the same frequency for a long period of time.
Japanese Patent No. 2984588 proposes a procedure for forming a frequency hopping pattern in which, even though adjacent interrogators use the same frequency at a certain period of time, frequencies used in the interrogators can be made different from each other by a hopping operation.
However, a frequency hopping pattern formed by a conventional forming procedure is a pattern using “(prime number)−1” frequencies. When the number of frequencies to be used is limited to “(prime number)−1”, all allocated frequencies may not be able to be used.
For example, in International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 18000-6, an international standard using a frequency band of 860 to 960 MHz in an RFID system is created. However, frequencies to be used change depending on nations or regions. In Japan, a UHF band of 952 to 954 MHz (1 to 9 channels) or 952 to 955 MHz (1 to 14 channels) is allocated.
Therefore, it is desired that, even though a frequency hopping pattern in which the number of frequencies to be used is not “(prime number)−1” is used, two adjacent interrogators are prevented from continuously using the same frequency for a long period of time.